1. Field of the Invention:
This invention relates to methods and apparatus for synthesizing an electrical output waveform from an essentially sinusoidal, multiphase AC source voltage as a function of a reference voltage waveform utilizing static power circuits. More particularly, the invention relates to techniques for sequentially turning on the static power switches and, specifically, to stabilizing integral control of the power switch firing signals.
2. Prior Art:
Static power frequency changers take several forms, however, the best known are probably cycloconverters which produce AC power at a desired frequency from a source of AC power of a different or varying frequency, phase-controlled converters which convert an AC voltage to a controllable DC voltage and AC motor controls which generate an AC voltage of varying frequency from a constant frequency source voltage. A more thorough discussion of the various types of static power frequency changers and their operating characteristics is set forth in a book entitled Static Power Frequency Changers by L. Gyugyi and B. R. Pelly, Wiley-Interscience, 1976. The common characteristic of all of these static power frequency changers is that they synthesize an output waveform of a desired frequency and amplitude from a multiphase, essentially sinusoidal AC voltage of a different frequency and generally a constant amplitude. This is accomplished by generating a plurality of component waveforms from selected portions of the individual phases of the source voltage. Static power switches connected in the several phases of the source voltage are rendered conductive by a pattern of signals developed by a control circuit in such a manner that the sum of the component waveforms produces an output waveform with a mean signal level which follows a reference voltage.
Suitable power switches for use with static power frequency changers may be of the type, such as analog switches, which remain conductive only as long as a control signal is applied to them, or they may be switches such as the thyristor (also called an SCR) which are rendered conductive by a firing pulse and remain conductive until the forward current is terminated. Thyristors may be arranged in static frequency changer circuits for natural commutation, that is, the firing of the thyristors is so arranged that each successive thyristor is commutated off by the phase voltage generated by the firing of the next successive thyristor, or they may be so arranged that forced commutation off by use of additional circuitry is required.
Regardless of the type of static power switches utilized in the frequency changer, the firing signals which render the switches conductive must be generated at properly phased intervals in order to produce a quality output waveform. One method of producing these firing signals is the cosine wave crossing technique which is described in Static Power Frequency Changers, at pages 289 to 298 and in Thyristor Phase-Controlled Converters and Cyloconverters, Pelly, Wiley-Interscience, 1971, at pages 229-241. In accordance with this technique, cosine timing waves derived from, and synchronized to, the multiphase source voltage are compared with the reference voltage to produce a firing signal as the reference voltage signal level becomes equal to the signal level of each successive cosine waveform. In theory, this technique produces a very high quality output waveform. However, because this technique depends upon instantaneous signal levels, noise, such as spikes or commutation notches on the cosine timing waves, can significantly advance or retard the individual firing instants thereby introducing distortion into the output waveform. Distortion will also result in the power circuit output due to distortion and commutation notches in the source voltage. In practice it is known to stabilize the cosine timing wave control with a feedback signal which, in some instances, takes the form of an integral of the output waveform.
Another method of generating firing signals for the power switches in a static power frequency changer is the integral control technique. This technique is described in Static Power Frequency Changers at pages 298 to 308, in Thyristor Phase-Controlled Converters and Cycloconverters, at pages 242 to 245 and in U.S. Pat. No. 3,585,485, Gyugyi, et al. In integral control, the difference between the output waveform and the reference voltage, known as the ripple voltage, is integrated. Each time this integral returns to zero which indicates that the mean value of the output waveform component for the given conduction period is equal to the average value of the reference voltage over the same period, a firing signal is generated transferring conduction to the next power switch. Since the average values of waveforms used for timing the firing pulses are utilized, the system is insensitive to input line voltage distortions and noises. At the same time, the integration periods are of short duration so that the system responds quickly to changes in the reference voltage. The basic integral control system provides satisfactory results with DC reference voltages; however, instability and loss of control occurs when the basic system is used with an AC reference voltage. As explained in U.S. Pat. No. 3,585,485, when the basic integral control is used with a converter having separate groups of thyristors, such as two, three pulse converters for a six phase voltage source, application of a sine wave reference voltage to the control results over time in advacement of the firing pulses in one group and retardation of the firing pulses in the other group. This introduces considerable distortion in the output waveform and could lead to complete loss of control.
Analysis reveals that this divergence of the firing angles between the two groups of thyristors, or half converters as they are referred to, is the result of DC components of opposite polarity appearing in the output waveforms of the half converters. U.S. Pat. No. 3,585,485 discloses a technique wherein the individual half converter waveforms are summed in opposition to the reference waveform and the resultant signal is integrated to extract the average DC component. The output of the main integrator is then compared separately with the average DC component from each half wave converter to generate firing signals for each half converter which are appropriately phase shifted by an amount which compensates for the DC component. Since the compensation is proportional to the error, the system can settle into a stable state.
While stabilization of the basic integral control for static power frequency changers in accordance with the teachings of U.S. Pat. No. 3,585,485 provides a quality output waveform, that technique requires an appreciable amount of hardware. In addition to the main integrator, an integrator, a comparator and the associated circuitry are required for each group of power switches. In order to generate a three phase output waveform from a six phase source voltage using two, three pulse groups of switches for the positive and negative banks associated with each output phase, twelve such additional circuits are required.
It is a primary object of the present invention to provide a quality output waveform from a static power frequency changer using a minimum of hardware.
It is also an object of the invention to achieve the above object in a manner and with apparatus which can be used with various kinds of static power frequency converters.
More particularly, it is an object of the invention to provide stable, integral control for static power frequency changers with a minimum of hardware.